WhatWeb can be stealthy and fast, or thorough but slow. WhatWeb supports an aggression level to control the trade off between speed and reliability. When you visit a website in your browser, the transaction includes many hints of what web technologies are powering that website. Sometimes a single webpage visit contains enough information to identify a website but when it does not, WhatWeb can interrogate the website further. The default level of aggression, called ‘passive’, is the fastest and requires only one HTTP request of a website. This is suitable for scanning public websites. More aggressive modes were developed for in penetration tests.

Most WhatWeb plugins are thorough and recognise a range of cues from subtle to obvious. For example, most WordPress websites can be identified by the meta HTML tag, e.g. ‘‘, but a minority of WordPress websites remove this identifying tag but this does not thwart WhatWeb. The WordPress WhatWeb plugin has over 15 tests, which include checking the favicon, default installation files, login pages, and checking for “/wp-content/” within relative links.